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October 2008

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From:
"David D. Hillman" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Mon, 27 Oct 2008 08:33:56 -0500
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Hi Ben - ah, now I see where you are going. Unfortunately, some of our 
industry "tribal knowledge" is need of a refreshing. The old rule of thumb 
was that if you were able to disperse the gold uniformly across the solder 
joint, you were safe and the issue of gold embrittlement was a closed 
topic. When we began using ENIG pwb surface finishes in much greater 
quantities, we discovered that the gold would migrate back to the 
solder/nickel solder joint interface and if the conditions were favorable, 
you could form a brittle gold/tin IMC that would degrade the solder joint 
(HP first reported this back in the early 90s). A bunch of good research 
(there have been a number of good articles in the Journal of Electronic 
Materials over the last 3 years) has demonstrated that the reason for the 
gold to migrate back to the solder/nickel interface is that a gold/nickel 
IMC phase is thermodynamically favored over a gold/tin IMC phase 
(sometimes material science isn't our friend). So, if you are soldering to 
a copper surface (aka immersion silver or immersion tin) the old tribal 
knowledge rules hold true. However,  if you are soldering to a nickel 
surface (aka ENIG) then you need to understand how much gold you are 
distributing across the solder joint and impact of the use environment 
time/temperature interactions. The smaller the solder joint, the greater 
the risk of issues (i.e. CSPs versus BGAs). The Binghamton paper that 
Richard recommended is a good paper to assist in your assessment. Good 
Luck.

Dave



"Gumpert, Ben" <[log in to unmask]> 
10/27/2008 06:27 AM

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Subject
RE: [TN] Au embrittlement on Copper vs Nickel






Dave,
 
I guess I wasn't clear. I'm not concerned with the Sn/Cu and Sn/Ni IMCs 
that initially form at the interfaces of the joint. I'm concerned with the 
ternary IMCs containing AU (such as Au0.5Ni0.5Sn4) that can form over 
time/temp along the joint interface.
 
reference:
http://physics.binghamton.edu/pub/cottsectc.pdf
 
This paper indicates that there can be long term effects to the solder 
joint, even if there is less than 3-4 wt% Au.
 
Ben

From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Friday, October 24, 2008 3:12 PM
To: TechNet E-Mail Forum; Gumpert, Ben
Subject: Re: [TN] Au embrittlement on Copper vs Nickel


Hi Ben - the major difference between the Au/Sn intermetallic (IMC), the 
Sn/Cu IMC and the Sn/Ni IMC is the structure of the IMC itself. All IMCs 
are brittle materials in comparison to solder joints. Both the Sn/Cu and 
Sn/Ni IMCs form as coherent structured layers at the interface between the 
pwb plated surface and the solder joint. The Au/Sn IMC is a needle-like or 
block-like structure that is located in the solder joint matrix or forms 
at the pwb plated surface/solder joint interface then intrudes into the 
solder joint matrix.  That explanation is very very simplified (and yes, 
there are some other forms of the Sn/Cu and Sn/Ni I am leaving out) but it 
gives you a relative idea on how the IMC structures differ. These IMC 
structure differences result in very different cracking modes.  Also, you 
must form either the Sn/Cu or Sn/Ni IMC to have a proper solder joint. The 
Au/Sn IMC forms only when you get approximately 3-4 wt% Au in the solder 
joint matrix and is not necessary for good solder joint formation.  Hope 
this helps. 

Dave 


"Gumpert, Ben" <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]> 
10/24/2008 01:03 PM 

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"Gumpert, Ben" <[log in to unmask]>



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Re: [TN] Au embrittlement on Copper vs Nickel








Dave,

I'll see if I can track down a copy of the AWS Soldering Handbook, but
the IPC-STD-001 HDBK doesn't go into the detail that I'm looking for.

Are you differentiating between the brittleness caused by AuSn4 in the
bulk solder and the brittle intermetallic layer that tends to form with
aging of the joint? I can understand the bulk solder brittleness being
the same, but are you saying that the effect of Au on the strength of
the intermetallic layer is not significantly different?

Ben


________________________________

From: [log in to unmask] [mailto:[log in to unmask]]

Sent: Friday, October 24, 2008 11:29 AM
To: TechNet E-Mail Forum; Gumpert, Ben
Cc: [log in to unmask]
Subject: Re: [TN] Au embrittlement on Copper vs Nickel



Hi Ben - gold embrittlement is a function of the amount of gold in the
solder joint and not dependent on the pwb pad being copper or nickel.
The IPC-STD-001 Handbook has a very good and short tutorial on gold
embrittlement in section 5.4.1.1 on page 63/64 which would be useful.
Also, there is good coverage of the topic in the AWS Soldering Handbook,
section 2.2 Contamination, pages 182-183. FYI - the AWS Soldering
Handbook, ISBN 0-87171-618-6, edited by Dr. Paul Vianco, Sandia National
Labs, is one of those references that everyone must have on their desks
(IMHO). It covers a huge range of solder and soldering process topics in
a well written format. 

Dave Hillman 
Rockwell Collins 
[log in to unmask] 



"Gumpert, Ben" <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]> 

10/24/2008 09:58 AM 
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Gumpert, Ben" <[log in to unmask]>


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Subject
[TN] Au embrittlement on Copper vs Nickel

 




Technetters,

Anyone care to share some comments on Au embrittlement?
Specifically, what is the relative difference between joints on copper
vs nickel? (For a SnPb joint)

Does Cu inhibit AuSn4 from forming in the bulk solder? Does this have
any effect on Au in the intermetallic layer?
Do the intermetallic compounds containing Au of each joint result in a
similar brittleness?
Do the joints have the same propensity for growth of intermetallic
compounds containing Au?

Any thoughts are welcome.
Ben



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